1. Field of the Invention:
This invention relates to an exterior protective member made of austenitic stainless steel for a sheathing heater element (heater) which is superior in oxidation resistance and anti-stress corrosion cracking and which has good weldability.
2. Prior Art:
In a so-called sheathing heater, a resistance heating element is covered, for protection, by a tubular protective member through a heat resistance electric insulating material, such as magnesium oxide. The protective member is hermetically sealed by means of welding. Since materials for such an exterior protective tubular member are required to have superior properties in oxidation resistance; resistance to stress corrosion cracking; and, also, good weldability, heretofore, high Ni heat resistance alloys such as NCF 800 and the like have widely been used.
High Ni heat resistant alloys of this type, however, have been found to have some drawbacks with respect to their insufficient performance in workability, weldability and too high a production cost for commercial use, though they have been proven to be considerably superior with respect to their resistance to oxidation as well as to stress corrosion cracking as explained above.
In order to obviate such drawbacks, as mentioned above, various proposals have been made.
For instance, Japanese Patent Publication No. 55(1980)-29146, proposes a protective member for electric heating members fabricated of a Fe-Ni-Cr alloy containing Cr, Ni, Si and Ce in a controlled and interrelated percentage with the intention to improve the alloy with regard to oxidation resistance property, resistance to stress corrosion cracking, weldability, but with reduced production cost.
Another proposal, Japanese Unexamined Patent Publication No. 48(1973)-13213, proposes a stabilized austenitic alloy steel free from forming welding cracks, and which aims at an economical production thereof by means of adjusting the content of Mn and Ti within a specified ratio depending on the amount of sulfur and phosphorus, respectively.
On the other hand, Japanese Patent Publication No. 57(1982)-19182 discloses an alloy having superior high temperature strength as well as good oxidation resistant property.
However, the protective member for an electric heating element disclosed by aforesaid Japanese Patent Publication No. 55-29146, has proven to be insufficient in oxidation resistance due to its low Cr content.
Speaking of the austenitic alloy steel disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 48(1973)-13213, although there is found no disclosure therein about the properties, such as, oxidation resistance and resistance to stress corrosion cracking, the steel is supposed to be beyond expectation in its resistance to oxidation in view of the fact that the steel does not have added to it rare earth metals such as Ce or the like.
As to the alloys disclosed by the Japanese Patent Publication No. 57(1982)-19182, they have drawbacks with respect to weldability because of the fact that they contain too high an amount of Si.
By taking the above-mentioned drawbacks into account, the present invention aims to provide an exterior protective member for a sheathing heater made of austenitic stainless steels superior in the properties of oxidation resistance, resistance to stress corrosion cracking, weldability and yet can be fabricated with low production cost.